Low energy contacts

ABSTRACT

High silver content (30 percent, by weight, or greater) gold base alloys are found to have exceptional resistance to silver sulfide tarnish when alloyed with from 0.25 to 10 percent, by weight, indium or preferably indium and cadmium each within the range of 0.25 to 10 percent and such alloys including those containing cadmium alone have been found to exhibit exceptional resistance to carbon film deposits in normal low energy switch applications and frictional polymer formation in sliding low energy applications.

O United States Patent [151 3,661,569 Abbott [451 May 9, 1972 54] LOWENERGY CONTACTS 2,400,003 5/1946 Henseletal. ..75/165x [72] Inventor:William H. Abbott, Columbus, Ohio primary Emminer L Dewayne Rutledge[73] Assignee: Battelle Memorial Institute, Columbus, Amsmm Hammer-JDay's Ohio Attorney-Gray, Mase and Dunson [22] Filed: June 19, 1969 [57]ABSTRACT App]. No.: 834,796

[52] U.S.Cl ..75/l65, 75/134 T, 75/151, 75/173 R, 200/l66 C [5 1] Int.Cl ..C22c 5/00 [58] FieldofSearch ..75/165, 134T, 151, 173 R; 200/ l 66C [56] References Cited UNITED STATES PATENTS 2,371,240 3/1945 Hensel etal. ..75/ l 65 High silver content (30 percent, by weight. or greater)gold base alloys are found to have exceptional resistance to silversulfide tarnish when alloyed with from 0.25 to 10 percent, by weight,indium or preferably indium and cadmium each within the range of 0.25 to10 percent and such alloys including those containing cadmium alone havebeen found to exhibit exceptional resistance to carbon film deposits innormal low energy switch applications and frictional polymer formationin sliding low energy applications.

7 Claims, 4 Drawing Figures FLOW RA FILM THICKNESS, A o

: 795 mlllmln.

TEST TIME HOURS lOO I000 Tornish Rules Of Au-Ag Alloys In Flowers 0fSulfur At 30 C IPATE'NITEDMAY 9 I972 SHEET 1 UF 4 Au-35 Ag Au-|9Ag IOOTEST TIME HOURS m i /Z 2 .1 I4 .m. r 0 m w m n.

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Tornish Rates Of Au-Ag Alloys In Flowers Of Sulfur Ai 30 C Fig. I

PATENTEDMAY 9:912 3,661,569

SHEET 2 [IF 4 Au-36Ag-2Cd-2In )IC/ Au-35Ag l6 loo Au-37Ag-8In 22 w 24Au-37Ag-4Cd-4In ALLOYS TESTED IN 50 PERCENT COLD- WORKED couomou.

m I IWI I I I II I I I I I0 I00 I000 |o,ooo

TEST TIME, HOURS EQUIVALENT FIELD EXPOSURE, YEARS Tarnish Rates Of,Au-Ag Alloys In Flowers Of Sulfur A1 30 C Fig. 2

LOW ENERGY CONTACTS BACKGROUND This invention relates to improvedelectrical contact materials and relates in particular to a new andnovel gold base alloy which exhibits improved characteristic of lowenergy electrical contact applications.

The metal terminal points of switching devices that are periodicallybrought together or separated to complete or interrupt electricalcircuits are subjected to a wide variety of mechanical, electrical, andchemical environmental conditions that require a correspondingly widevariety of properties to effectively withstand such conditions. An idealalloy for a particular application is one that does not erode, melt, orfuse in the presence of the electrical resistive and arc heat, provideslow contact resistance between the contacting surfaces, does not formcurrent inhibiting films (oxides, sulfides, polymers, carbon deposits,etc.), possesses sufficiently high hardness to provide wear resistance,is sufficiently ductile for fabrication, and is sufficiently low in costto, be feasible for the application.

Where high voltage and/or amperage circuits are involved, high meltingpoint materials are generally required frequency at the sacrifice ofother properties such as corrosion resistance. Where low-voltage,low-amperage applications are involved, such as microcircuits, telephonerelay switches, etc., the most satisfactory alloys are the preciousmetal contact materials such as gold, silver, platinum, palladium, andalloys thereof. Such metals and their alloys possess relatively lowmelting points but where the potential is less than about 50 volts andcurrent flow is less than about 1% amp., these materials may besuccessfully employed. Their chief value in such applications relateslargely to their resistance to corrosion. Switching devices made ofthese materials are capable of long time reliability even in thepresence of relatively corrosive environments. However, sticking, wear,erosion, and corrosion continue to be a problem.

One of the most significant metals for low-voltage low-amperageswitching applications is gold and its alloys. Gold is unsurpassed inits resistance to oxidation and sulfidation but possesses a relativelylow melting point, is susceptible to erosion and is costly. Also currentretarding carbon and polymer deposits tend to form on the contactingsurface of gold (and its alloys) when utilized as electrical contactpoints where organic vapors are present. Such vapors are generallypresent in switching devices of this type emanating from surroundinginsulating materials, switch lubricating greases, etc.

Elemental gold is too soft for many electrical contact applicationsparticularly where the switch application involves frequent opening andclosing so that frictional wear is substantial. However, it is possibleto make relatively small additions of alloying constituents such assilver to effect some hardenmg.

Silver possesses many qualities that surpass those of gold. For example,this metal has the highest electrical and thermal conductivityproperties of any metal or alloy and is reasonably. resistant tooxidation in air. Additionally, silver is in far greater supply thangold and consequently is far more economical to use. However, silver isparticularly susceptible to sulfur attack forming sulfide films of athickness that creates significant electrical contact resistance.

A frequently employed alloy for low energy (low-voltage low-amperage)switching applications consists of gold-silver alloys consisting ofabout 5-30 percent, by weight, silver balance essentially gold. Thesilver addition hardens the gold and reduces the overall costs. However,as the silver contents rise, the tarnish resistance of the alloydecreases, particularly as it relates to the formation of sulfides. Atthe lower silver contents, the alloy is susceptible to carbon andpolymer deposits from organic vapor.

The elements indium and cadmium are known additions to gold for variouspurposes. For example, in the alloy of (1.8. Pat No. 2,400,003, indiumadditions to gold alloys containing up to 24 percent silver are made toreduce the metal transfer tendency of these compositions. In the alloyof US. Pat. No. 2,806,113, the addition is made to reduce the formationof metal dust due to mechanical abrasion and the formation of burnishedlayers. In all of the compositions taught by these and other prior artpublications, the recognized compositions contain less than 30 percent,by weight, silver. The reason for this, of course, is that compositionscontaining greater amounts of silver exhibit corrosion resistance andparticularly resistance to the formation of sulfide films that is morelike silver than gold while simultaneously exhibiting a susceptibilityto.carbon and polymer deposits.

THE INVENTION 1 have now found that gold alloys having high silvercontent (i.e., 30 percent, by weight, or greater) exhibit unexpectedlyhigh resistance to sulfide film formation when alloyed with from 0.25 to10 percent, by weight, indium and particularly when further alloyed withfrom 0.25 to 10 percent, by weight, cadmium.

I have further discovered that these alloys including those containingcadmium alone (Au-30 percent or greater, Ag- 10 percent Cd) aresuprisingly resistant to .the formation of current inhibiting carbonfilms during normal contact applications and polymer film formationduring sliding contact applications.

Optimum results are experienced by making indium plus cadmium alloyingadditions to gold base alloys containing 30 to 60 percent, by weight,silver.

THE DRAWINGS The unexpectedly high tarnish and carbon film-resistanceproperties of the alloy of the present invention are best illustrated bythe data of the tables and the drawings wherein:

FIG. 1 is a graph showing the silver sulfide tarnish rates of binarygold-silver alloys;

FIG. 2 is a graph showing the silver sulfide tarnish rates of theelectrical contact alloys of the present invention;

FIG. 3 is a graph showing the contact resistance of the present contactalloys as applied to normal switch applications and illustrates theirresistance to carbon film formation; and

FIG. 4 is a graph showing the contact resistance and coefficient offriction of the present contact alloys as applied to sliding switchapplications and illustrates their resistance to adverse polymerformation.

I DETAILED DESCRIPTION When silver is exposed to sulfur-containingatmospheres (free or elemental sulfur), a tarnish film of Ag,S forms onits surface. This film grows in accordance with time and otherenvironmental conditions (particularly the availability of free orelemental sulfur) until it constitutes an electrical current barrier.Where the sulfur content of the surrounding atmosphere is relativelyconstant, as when exposed to ordinary atmospheres, the film grows.

This film also forms on the surface of Au-Ag alloys. However, when lowsilver content, Au-Ag alloys, are exposed to free sulfur-containingatmosphere, the Ag s tarnish film forms at a slower rate than on puresilver and its growth rate depends on the silver content. Where thesilver content of the alloy exceeds about 30 percent, by weight, the AgS film grows at a rate equivalent to that of pure silver until anintermediate film thickness of from about 30 to 40 angstroms is reachedwhereupon a retarded growth rate occurs depending on the exact goldcontent. The combined tarnish film growth is sufficient to reduce theattractiveness of the alloy. Consequently, gold-base alloys containingmore than about 30 percent, by weight, silver have not been extensivelyutilized in the past.

This phenomenon is best illustrated by the data of FIG. 1 of thedrawings. The curves 10, l2, 14, 16, and 18 represents Ag,S filmthicknesses (in angstroms) of specimens of pure silver 10 and alloys ofincreasing gold (or decreasing silver) content (curves 12, 14, 16, and18) after exposure to'a sulfurcontaining atmosphere (nitrogen-20 partsoxygen-40 parts per billion free sulfur).

These accelerated sulfur tarnish tests consists of positioningrod-shaped specimens (0.100 in. X 1.500 in.) in a glass reaction chamber(200 mm Scheibler desiccator) and introducing the specified gas flowthrough a glass tube extending downward (vertically) from the center ofthe chamber to within it inch of its bottom. The specimens arepositioned symmetrically around the glass-inlet tube at a 45 degreeangle and in a single layer. The distance from the center and bottom ofthe desiccator to the center of the rods were 7.5 and 10.0 centimeters,respectively. Testing was conducted at a temperature of 30 C. 1- C. Thegas was purified prior to use by passage through a l X 18 inch molecularsieve column. The flow rate was 795 :t ml/min. The gas base was N, 200mixture towhich sulfur vapor was added by saturating the mixture intemperature controlled columns. The sulfur vapor concentration wascalculated and extrapolated from known data.

It will be noted that each of the curves of FIG. 1 (except curve 18)intersect the curve 10 for pure silver at a definite Ag,S thickness.These values are in the range of 25-40 A for the 35, 56, and 83 percent,by weight, Ag alloys (curves l6, l4, and 12) and 5-6A for the Au-l9 Agalloy. Thus, it is readily discernible that the overall tarnishprocesses of Au-Ag alloys involves: (1) an initial linear rate oftarnish that is equivalent to that of pure silver; and (2) a secondaryor lower rate of film (Ag,S) formation.

I have found that the addition of from 0.25 to 10 percent, by weight, ofindium or indium plus 0.25 to 10.0, by weight, cadmium to thegold-silver alloys containing 30 percent or more silver, andparticularly where the silver content is within the range of from 30 toto 50 percent, by weight, materially alters the secondary or lower rateof Ag,S tarnish film formation.

This phenomenon is best illustrated by the data of FIG. 2. Curves l0 and16 of FIG. 2 are curves 10 and 16 of FIG. 1 showing the corrosion ortarnish film formation of Ag,S of pure silver and Au-35 percent Agalloy. Curves 20, 22, and 24 show the corrosion rate or tarnish filmformation of goldsilver alloys containing greater than 30 percent, byweight, silver and varying indium contents within the range of 0.25 to10 percent, by weight, as set forth in Table I below:

TABLE I. Tarnish Characteristics of Au-Ag Alloys (a) 50 percentcold-worked condition. (b) Tested to only I hours. (c) Compositionsestimated in drawings.

The data of Table I and FIG. 2 show conclusively that indium, andparticularly the combination of cadmium plus indium additions,significantly decreases the tarnish rate of the Au- 30 percent, byweight, or more Ag alloys. The table and graph show that the ratedecrease "is generally attributable to decreases in the secondarycorrosion rates. For example,

curve 16 shows a transition thickness of 32 A and a corrosion rateexponent (n of 3 for Ag34.5 Ag alloy while substantially the same alloycontaining 3.87 percent, by weight, In exhibits a similar transitionfilm thickness (35A) but a much higher rate exponent (over 5).

The rate exponent relates to the secondary tarnish rate as follows:

X" kt X X transition where X film thickness, A

t= time, hours n rate exponent l k constant Thus, the larger theexponent n the smaller the film thickness X for any given period of timet. For binary Au-Ag alloys containing up to 35 weight percent silver,values of n =3 were obtained indicating that the secondary stage oftamishing followed a cubic rate equation. Silver contents above 35percent, give progressively lower values of n eventually reaching n forpure silver. Consequently, the curves 20, 22, and 24 of FIG. 2 and thedata of Table I show that the addition of indium and indium plus cadmiummaterially increase the exponent indicating a significantly slowersecondary tarnish rate.

Carbon deposits on the surface of electrical contacts are related to thepresence of organic vapors and electrical arcing during opening andclosing of electrical switches commonly referred to as make and breaktype contacts. This problem is not related to sliding and may occurwhere only normal contact loading is involved. Accordingly, the alloysof the present invention were tested .in a controlled environmentutilizing two 0.10 inch diameter X 1.0 inch long rod-shaped specimens aselectrical contact points. These rods were crossed in a horizontalposition-one being in a fixed position and the other being verticallypositionable to effectnormal make and break contacts with the fixed rod.When the rods were in contact with one another, a contact force of about2.5 grams was maintained. The make-and-break cycle rate was one completecycle every two seconds (1.1 seconds closed and mated and 0.9 secondsopen).

The organic vapors commonly present during switching operations are thesolvents from the resinous insulating materials. Such solvents are oftenaromatic compounds such as benzene. Consequently, the controlledenvironment of the present tests'consisted of a nitrogen-benzene mixture(N,- 560 ppm C,I-I,, 250 cc/min at 760 mm Hg).

When carbon deposits occur on the surface of make-andbreak contacts, thecontact resistance rises sharply. This phenomenon or the point of carbonformation is referred to as activation since arcing increases '(andconsequently an even greater carbon deposit occurs). Curve 30 of thegraph of FIG. 3 illustrates activation" or excessive carbon formation onbinary Au-Ag alloys containing up to about 60 weight percent silverexposed to the aforementioned make-and-break testing at 3 volts directcurrent and I00 milliamperes during about 50,000 cycles of switching.The alloys of the present invention (curves 22, 24, and 26 representingAu-37Ag-8In, Au-37Ag- 4Cd-4ln, and Au-37Ag-8Cd, respectively, resistedactivation for over 50,000 cycles. The Au-37Ag-8ln gave the most stablecontact resistance-life characteristics of 7 to 10 milliohms. Arelatively large resistance decrease was obtained for the Au- 37Ag-7.8Cdalloy during the first 1,000 cycles of switching after which theresistance remained stable (3-3.5 milliohms). Although higher values(IO-l3 milliohms) were reached by the Au-37Ag-4Cd-4In alloy these valuesare exceptionally good for a gold-base alloy.

Photomicrographic examination of the contact areas of the Au-37Ag-8Cdand Au-37Ag-8In alloys after 40,000 cycles showed no evidence of organiccontamination.

Thus, it is apparent from the data of FIG. 3 that the alloys of thepresent invention in addition to unusual sulfide tarnish resistance areresistant to activation" or carbon film formation in low energyapplications.

A still further advantageous feature of the present alloys relates totheir unusual resistance to frictional polymer formation in slidingcontact applications. Such polymers are known to form where organicvapors are present and the contact is made in a manner where frictionalheat is generated such as in slip ring electrical contacts. Thephenomenon is independent of arcing and will form independently of anactual electric current.

Testing designed to simulate average field applications involvedcrossing 0.1 inch diameter X 1.5 inch test rods at 90 as describedabove. However, instead of make-and-break cycles one rod was caused toslide over the surface of the other in violin fashion (at a constantreciprocal velocity over a 1 cm length wear tract). The fixed rod(called the rider) was actually horizontally movable with thereciprocating rod but offered predetermined resistance. The actualdistance the rider moved horizontally was translated into coefficient offriction values. A cycle was completed every two seconds (l cm/sec.).The contact (or normal) force was maintained constant for all tests. Thespecimens exhibited substantially uniform surfaces (a profilometerreading of about 20 microinches R.M.S.).

Although polymer formation is generally regarded to be undesirable, theexact type of polymer that forms an electrical contact varies inaccordance with the material from which the contact is constructed andthe organic vapor that is present. Some such films actually act aslubricants decreasing friction without materially changing contactresistance (noise). However, many drastically increase noise. Curve 50and 50a of the data of FIG. 4 shows the coefficient of fraction andcontact resistance values, respectively for Au-37Ag alloy. The drop incoefficient of friction of about 50,000 cycles is attributable to thelubricating qualities of the polymer. This drop in coefficient isaccompanied by a sharp increase in noise.

Curves 22, 24, and 26; 22a, 24a, and 26a relate to Au- 37Ag-8ln,Au-37Ag-4Cd-4ln, and Au-37Ag-8Cd compositions, respectively. These datashow excellent contact-resistance stability and low friction in bothnitrogen and Ng-cgl'ig environments (nitrogen gas containing benzenevapor). Polymer formation occurred only on the Au-37Ag-7.8Cd alloy(curve 52) and this polymer was of a type that lubricated the contactingsurfaces to give even lower friction values (p 0.44 without raisingcontact resistance.

What is claimed is:

1. An alloy consisting of from 30 to 60 percent, by weight, silver, 0.25to 10 percent, by weight, of at least one element selected from thegroup of cadmium and indium, balance essentially gold.

2. The alloy of claim 1 wherein both cadmium and indium are present,each within the range of 0.25 to 10 percent, by weight.

3. A gold-base alloy consisting of from 30 to 60 percent, by weight,silver, from 0.25 to 10 percent, by weight, indium, balance essentiallygold.

4. An electrical contact formed of an alloy that consists essentially offrom 30 to 60 percent, by weight, silver, 0.25 to 10 percent, by weight,of at least one element selected from the group cadmium and indium,balance gold, said contact having the properties of high resistance tocarbon film build up on contacting surfaces in normal low energyapplications and high resistance to frictional polymer formation in lowenergy applications.

5. The contact of claim 4 wherein the alloy contains both cadmium andindium each within a range of from 0.25 to 10 percent, by weight.

6. The contact of claim 4 wherein the alloy contains indium and has theproperty of a high resistance to silver sulfide tarnish.

7. The contact of claim 4 wherein the alloy contains indium plus cadmiumand has the property of a high resistance to silver sulfide tarnish.

2. The alloy of claim 1 wherein both cadmium and indium are present,each within the range of 0.25 to 10 percent, by weight.
 3. A gold-basealloy consisting of from 30 to 60 percent, by weight, silver, from 0.25to 10 percent, by weight, indium, balance essentially gold.
 4. Anelectrical contact formed of an alloy that consists essentially of from30 to 60 percent, by weight, silver, 0.25 to 10 percent, by weight, ofat least one element selected from the group cadmium and indium, balancegold, said contact having the properties of high resistance to carbonfilm build up on contacting surfaces in normal low energy applicationsand high resistance to frictional polymer formation in low energyapplications.
 5. The contact of claim 4 wherein the alloy contains bothcadmium and indium each within a range of from 0.25 to 10 percent, byweight.
 6. The contact of claim 4 wherein the alloy contains indium andhas the property of a high resistance to silver sulfide tarnish.
 7. Thecontact of claim 4 wherein the alloy contains indium plus cadmium andhas the property of a high resistance to silver sulfide tarnish.